Delta Cephei

Delta Cephei (δ Cep / δ Cephei) is a binary star system approximately 891 light-years away in the constellation of Cepheus (the King). The names Alrediph, Al Radif or variants are derived from the Arabic "الرادف" (al-rādif), or "the Follower". Delta Cephei is the prototype of the Cepheid variable stars, and it is among the closest stars of this type to the Sun (with Polaris being closer). Its variability was discovered by John Goodricke in 1784, the second Cepheid variable discovered following Eta Aquilae earlier the same year.

Stellar system

Delta Cephei within the constellation of Cepheus.

Unlike the eclipsing binary Algol, Delta Cephei's variability is due to pulsation of the star. It varies from magnitude 3.6 to 4.3, and its spectral type also varies, from about F5 to G3. The period is 5.36634 days; rise to maximum is quicker than the subsequent decline to minimum. It was later discovered that there are two types of Cepheid variables, and Delta Cephei is now known as a type I (Classical) Cepheid.

Stars of this type are believed to form with masses of 3–30 times that of our Sun, and then have passed through the main sequence as B-class stars. With the hydrogen burnt up in their core, these unstable stars are now passing through later stages of nuclear burning.^[2]

Knowing the distance of Delta Cephei and others in its class is fundamental to calibrating their period-luminosity relationship; unfortunately, these efforts were until recently marred by only fair parallax accuracy. In 2002 however, the Hubble Space Telescope was used to determine the distance to Delta Cephei (and RR Lyrae, another standard candle) within ~4%: 273 parsecs, or 890 light-years.^[3] There is a 7.5 magnitude companion star, separated from Delta Cephei by 41 arc seconds and visible in small telescopes.

References

External links

• "Delta Cephei". American Association of Variable Star Observers. 2000-09. http://www.aavso.org/vstar/vsots/0900.shtml. Retrieved 2008-06-21. 
• "Delta Cephei". The Internet Encyclopedia of Science. http://www.daviddarling.info/encyclopedia/D/Delta_Cephei.html. Retrieved 2008-06-21. 
• "Delta Cephei, 27 Cephei". SEDS. http://www.seds.org/~spider/spider/Vars/deltaCep.html. Retrieved 2008-06-21. 
• Kaler, Jim. "Delta Cephei". University of Illinois. STARS. http://www.astro.uiuc.edu/~kaler/sow/deltacep.html. Retrieved 2008-06-21. 
• "δ Cephei". Alcyone ephemeris. http://www.alcyone.de/SIT/mainstars/SIT000496.htm. Retrieved 2008-06-21.